This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Data collection was completed for our study of the effects of serotonin on the firing properties of neurons in the spinal cord of behaving monkeys. New data confirm our findings in other monkeys from previous years. Serotonin and the serotonin receptor antagonist methysergide were applied to extracellularly recorded neurons with a combined extracellular, iontophoresis electrode. Serotonin causes overall increase in activity of a significant percentage of spinal neurons. A small percentage exhibit decreased firing after iontophoresis of serotonin. Methysergide has the opposite effect for most of these neurons. The remaining neurons are not affected by either serotonin or its antagonist. The directional tuning of neurons in the C6-T1 segments was determined for multi-dimensional wrist movements before and after the iontophoretic application of serotonin and serotonin antagonists. Block of inhibition had little effect on the neurons'directional tuning for the majority of neurons. These results suggest that serotonin plays an important role in regulating excitability for many spinal neurons but does not affect spatial tuning for wrist movements in different directions.